Acoustic liner

ABSTRACT

An acoustic liner comprising a sound permeable inside plate forming a first closed annulus, and a sound impermeable outside plate forming a second closed annulus located outside of and extending around the first closed annulus. The inside and outside plates are spaced apart and thus form an annular chamber therebetween; and a core member is secured in this annular chamber, between the inside and outside plates. The core member forms or has the shape of a sine wave form annularly extending around the inside plate, and the core member and the inside plate from a multitude of varying depth sound absorption chambers to attenuate sound waves over a broad band of frequencies.

BACKGROUND OF THE INVENTION

This invention generally relates to acoustic liners, and moreparticularly, to annularly or circumferentially shaped acoustic liners.Even more specifically, the present invention relates to a highefficiency broad band acoustic liner of the type especially well-suitedto line the interior of a duct or shroud of a jet engine.

Acoustic liners are employed in many applications to attenuate noisesgenerated by machinery or equipment; and, for instance, jet engines arealmost universally provided with sound absorption liners or panels toattenuate sound waves produced inside the engines. One type of soundabsorption liner commonly used in jet engines comprises a soundpermeable facing sheet, a sound impermeable backing sheet and ahoneycomb core interposed between these two sheets. Such devices aregenerally referred to as laminar absorbers, and one such absorber isdisclosed in U.S. Pat. No. 3,166,149.

These prior art panels are simple, strong and light weight, andheretofore have generally produced acceptable results. Governmentregulations limiting the level or amount of noise that may be emittedfrom a jet engine are becoming stricter, though, and it may be verydifficult for many common types of jet engines to comply with these morestringent noise limits using conventional prior art laminar soundabsorbers. A principle reason for this is that most laminar absorbersare able to absorb sound effectively only at certain discretefrequencies, and between these discrete absorption bands, the absorptionfalls to a very low level.

Various attempts have been made to broaden the frequency range overwhich laminar absorption panels effectively attenuate sound waves;however, heretofore these attempts have not yielded any commerciallypractical designs. For example, a broader sound absorptioncharacteristic may be obtained by providing the absorption panel withplural layers of permeable sheets and honeycomb cores, and examples ofprior art devices of this general type are shown in U.S. Pat. Nos.3,439,774; 3,640,357 and 3,670,843. These prior art broad band acousticliners are bulky and heavy, though, and are difficult to manufacture ina commercially practical manner. Another approach to increasing thefrequency range over which laminar absorption panels effectivelyattenuate noises involves modifying the shape and design of thehoneycomb structure, and examples of this approach are found in U.S.Pat. Nos. 4,421,201; 3,913,702 and 3,831,710. These attempts usuallyresult in a complex honeycomb design that also is difficult andexpensive to manufacture.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved soundabsorption panel that is simple and economical to fabricate and thatfunctions effectively over a comparatively wide range of absorptionfrequencies.

Another object of this invention is to provide an annularly shaped,broad band sound absorption panel that is simple and economical tomanufacture.

A further object of the present invention is to provide a highefficiency broad band acoustic resonator and absorption panel for a jetengine, that is simple to manufacture and is well suited for use on aretrofit basis, and that can be used in many conventional jet enginedesigns.

These and other objectives are attained with an acoustic linercomprising a sound permeable inside plate forming a first closedannulus, and a sound impermeable outside plate forming a second closedannulus located outside of and extending around the first annulus. Theinside and outside plates are spaced apart and thus form an annularchamber therebetween; and a core member is secured in this annularchamber, between the inside and outside plates. The core member has theshape of a sine wave form annularly extending around the inside plate,and the core member and the inside plate form a multitude of varyingdepth sound absorption chambers to attenuate sound waves over a broadband of frequencies.

Further benefits and advantages of the invention will become apparentfrom a consideration of the following detailed description given withreference to the accompanying drawings, which specify and show preferredembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a gas turbine engine including a pair of acoustic linersaccording to the present invention.

FIG. 2 is a front view of one of the acoustic liners.

FIG. 3 is an enlarged front view of a portion of the acoustic liner.

FIG. 4 is a further enlarged view of a portion of a core member of theacoustic liner, particularly showing the laminar construction thereof.

FIG. 5 is a top view of the portion of the core member illustrated inFIG. 4, with various layers partially broken away.

FIG. 6 is similar to FIG. 3 but also shows a bulk sound absorptionmaterial inside the acoustic liner.

FIG. 7 is similar to FIG. 3, but also shows a honeycomb structure heldinside the acoustic liner.

FIG. 8 is a cross-sectional view through the honeycomb structure, takenalong line VIII--VIII of FIG. 7.

FIG. 9 is similar to FIG. 2 and shows how the liner may be comprised ofa plurality of sections.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 outlines jet engine 10 generally comprising shroud or duct 12,fan 14, compressor 16, turbine 20 and acoustic liners 22 and 24. In aconventional manner, air is drawn into engine 10 through inlet 26 byrotating fan 14, and this air is compressed by compressor 16 and thenheated in a combustion chamber by the combustion of fuel. The heated airis expanded through turbine 20, driving the turbine, which in turn isused to drive fan 14 and compressor 16, and the heated and expanded airis discharged from the engine through outlet 30. The discharged air isat a much a higher velocity than the air drawn into the engine throughinlet 26, producing the desired thrust. Preferably, shroud 12, fan 14,compressor 16 and turbine 20 are of conventional construction andoperate in a conventional manner, and it is unnecessary to describethese elements further herein.

In the operation of engine 10, significant sound waves are produced bothin the forward and rearward sections of the engine. The sound waves inthe forward section of the engine are primarily generated by therotating fan 14, and typically the frequencies of these sound waves arewithin a relatively narrow band, with the central frequency of that banddetermined principally by the rotating speed of fan 14. The sound wavesin the rearward section of the engine are produced by compressor 16,turbine 20 and the high velocity of air moving through this area of theengine, and typically, the frequencies of these sound waves aredistributed over a relatively wide range in a highly irregular manner.

Acoustic liner 22 is secured within a forward area of engine 10 toattenuate sound waves generated in this area of the engine, and acousticliner 24 is secured within a rearward area of the engine to attenuatesound waves produced therein. Preferably, as shown in FIG. 1, liner 22extends rearward from a position adjacent inlet 26 to a positionimmediately forward of fan 14, and liner 24 extends forward from aposition adjacent outlet 30 to a location extending around air flowguides 32 of the engine. Liners 22 and 24 are generally identical, andthus only one, liner 22, shown in detail in FIGS. 2 and 3, will bedescribed herein in detail.

Liner 22 includes inside plate 34, outside plate 36 and core member 40.Generally, inside plate 34, commonly referred to as a facing sheet, issound permeable and forms a first closed annulus; and outside plate 36,commonly referred to as a backing sheet and which preferably is soundimpermeable, forms a second closed annulus that extends around and isspaced from the inside plate. The inside and outside plates thus form aclosed annular chamber therebetween; and core member 40 is secured inthis annular chamber, between plates 34 and 36. The core member forms asine wave form annularly extending around the inside plate; and in thisway, the inside plate and the core member form a multitude of varyingdepth sound absorption chambers 42 that effectively attenuate soundwaves over a broad range of frequencies. In particular, at each point ineach chamber 42, sound waves are attenuated in one or more frequencybands, each of which is centered around a particular frequencydetermined by the radial depth of the sound absorption at that point.Because the depth of each chamber 42 varies significantly, each chamberwill effectively attenuate sound waves over a relatively wide range offrequencies.

With the preferred embodiment of liner 22 shown in FIG. 2, inside plate34 and outside plate 36 both have substantially circular shapes, withthe inside plate radially located inside of and concentric with theoutside plate. Moreover, with this preferred liner 22, core member 40has a uniform wave length, over its entire circumference, with theinside peaks or edges of the wave form engaging the inside plate andwith the outside peaks or edges of the wave form engaging the outsideplate. In addition, liner 22 has a substantially cylindrical shape, withthe inside plate having a substantially uniform radius, r₁, over itsentire length, and with the outside plate having a substantially uniformradius, r₂, over its entire length. Further, the shape of core member 40is substantially uniform in the axial direction, so that the soundabsorption chambers comprise axial channels extending along the entirelength of the liner.

The inside plate 34 may be fabricated from metal, plastic, ceramic, orother suitable materials; and, for instance, the inside plate maycomprise a single discretely perforated metal sheet, or a combination ofsuch a metal sheet and a porous fibrous layer, or a porous compositeweave material bonded to a woven wire mesh. Depending on the specificenvironment in which the acoustic liner is used, it may be desirable toprovide the radially inside surface of the inside plate with a corrosionresistant coating. The outside plate 36 may also be fabricated frommetal, plastic, ceramic or other suitable materials; and for example,the outside plate may comprise a solid aluminum plate.

Core member 40 may be made from any suitable material such as plastic,paper, metal, ceramic or from a woven composite material, and forinstance, the core member may be fabricated from a flat sheet ofaluminum that is bent into the desired sine wave shape. With theembodiment of liner 22 illustrated in FIGS. 2 and 3, the core member isconstructed from a sound impermeable material, although, as discussedbelow, the core member may also be formed from a sound permeablematerial.

FIGS. 4 and 5 illustrate one preferred construction of the core member,in which this member is comprised of multiple layers 40a-e of acomposite material that, in turn, comprises epoxy reinforced carbonfibers 44. The fibers in each layer 40a-e are aligned in a particulardirection; and the individual layers are placed one on top of anotherwith the fibers of the different layers aligned in a variety ofdifferent directions to produce a composite material that has a highstrength in all directions. For example, the individual layers 40a-e ofcore member 40 may be formed in the preferred sine wave form and thensecured together to form the core member. It should be noted that, whileFIGS. 4 and 5 illustrate five individual layers, in practice it may bepreferred to form the core member 40 from more layers, such as tenlayers.

Core member 40 may be secured in the annular chamber between plates 34and 36 in any suitable manner, although preferably the radially insidepeaks or edges of the core member abut against and are secured to insideplate 34, and the radially outside peaks or edges of the core memberabut against and are secured to outside plate 36. The preferredtechnique for securing the core member in place generally depends on thematerial or materials from which that core member is made. For instance,if the core member is made from epoxy reinforced carbon fibers, then theinside and outside edges of the core member may be secured,respectively, to the inside and outside plates by an adhesive. If thecore member is made from aluminum, it may be bolted, welded ormechanically interlocked to the inside and outside plates of the liner22.

Various modifications may be made to the basic construction of liner 22shown in FIGS. 2 and 3 to improve the sound attenuation characteristicsof the acoustic liner. For example, with reference to FIG. 6, coremember 40 may be made from sound permeable material, and chambers 46,which are formed by the core member and outside plate 36, may be filledwith a bulk acoustic absorbing material 50. In this way, chambers 42 andchambers 46 of liner 22 are both used to attenuate sound waves. Anysuitable bulk acoustic material may be used, and for example, thematerial may be of the type identified by the trademark Kevlar.

Alternatively, as depicted in FIG. 7 and 8, sound absorption chambers 42may be filled with honeycomb structures 52. Preferably, the walls 54 ofeach honeycomb structure 52 radially extend completely between insideplate 34 and core member 40, and each channel 42 is filled with arespective one of the honeycomb structures. These structures, first,preferably prevent or inhibit sound waves from moving axially throughthe interior of liner 22, and second, strengthen the liner, both in theaxial and radial directions. Honeycomb structures 52 may have anycommonly used honeycomb core design and may be made of any commonly usedhoneycomb material, and for instance, the structures may have cell sizesin the range of 1/8 to 1/2 inch. Honeycomb structures 52 are preferablysecured to both inside plate 34 and core member 40, and this may be donein any suitable manner such as by an adhesive. In addition, if desired,the length of the sine waves formed by core member 40 may vary over thecircumference of the core member. For instance, this wave length may berelatively small over one portion of the core member, and comparativelylarge over another portion of the core member.

As previously mentioned, liner 24 is substantially identical to liner22. The principle differences between these liners relate to variousparameters, such as the radial thickness of core member 40, the wavelength of the sine pattern of the core member, and the specificmaterials from which the elements of the liner are made. As will beappreciated by those of ordinary skill in the art, these parameters areselected for each liner depending on the specific application in whichthe liner is used, and in particular, to help achieve the desired soundattenuation characteristics for the liner.

Acoustic liner 22 may be assembled and secured in jet engine 10 in anysuitable manner. With reference to FIG. 9, with one preferred technique,the liner is comprised of three sections 22a, b and c that are formedseparately and then connected together as they are placed in position inengine 10. Each of these liner sections includes a respective onesegment of inside plate 34, outside plate 36 and core member 40 so thatwhen these sections are connected together, they form the complete linerillustrated in FIG. 2. These liner sections may be secured in jet engine10 and to each other in any suitable procedure, such as by bonding,welding, bolts or by mechanical interconnections.

A principle advantage of liner 22 is that it is comparatively simple andinexpensive to manufacture. To elaborate, each section 22a, b and c ofthe liner can be made by simply forming a sheet of aluminum or othersuitable material into the desired sine wave shape to form a segment ofthe core member 40, and then placing this sine wave form betweensegments of the inside and outside plates. This procedure does notrequire any special cutting, notching or further shaping of the coremember and is not expensive or time consuming. At the same time, thistechnique produces the desired multiple, varying depth sound absorptionchambers. Moreover, this manufacturing procedure places very fewlimitations on various parameters of liner 22--such as the radialthickness of the core member and the specific materials from which thecore member and inside plate 34 are made--which may be changed to varythe sound attenuation characteristics of the liner, so that thisprocedure can be used to construct different liners that effectivelyattenuate sound waves over various, broad frequency ranges.

As described above, acoustic liners 22 and 24 have been described asbeing used adjacent the inlet and outlets of a jet engine. As will beunderstood by those of ordinary skill in the art, an acoustic linerembodying the present invention can be applied equally well to otherparts of a jet engine where noise attenuation is desired or required.Indeed, this invention is not restricted to jet engines, but may also beused in any duct in which gas is flowing, or for enclosing any space inwhich sound waves are generated.

While it is apparent that the invention herein disclosed is wellcalculated to fulfill the objects previously stated, it will beappreciated that numerous modifications and embodiments may be devisedby those skilled in the art, and it is intended that the appended claimscover all such modifications and embodiments as fall within the truespirit and scope of the present invention.

We claim:
 1. An acoustic liner comprising:a sound permeable inside plate forming a first closed annulus and defining a liner axis; a sound impermeable outside plate forming a second closed annulus located outside of and extending around the first closed annulus, the inside and outside plates being spaced apart and forming an annular chamber therebetween; a core member secured in the annular chamber, between the inside and outside plates, the core member extending axially along and around the liner axis and forming a sine wave form in the annular direction around said axis, said sine wave form extending completely around the liner axis, wherein the core member and the inside plate form a multitude of varying depth sound absorption chambers to attenuate sound waves over a broad range of frequencies; and a multitude of honeycomb structures located in the sound absorption chambers to further attenuate the sound waves; and wherein the core member has an axial length; and in any plane perpendicular to the liner axis and along said axial length, the core member forms a sine wave form having a generally uniform height and wave length.
 2. An acoustic liner according to claim 1, wherein each honeycomb structure radially extends completely between the inside plate and the core member.
 3. An acoustic liner according to claim 2, wherein each honeycomb structure is secured to both the inside plate and the core member.
 4. An acoustic liner according to claim 2, wherein:the closed annular chamber defines an axis; the core member forms a multitude of inside and outside axially extending edges; the outside edges of the core member abut against the outside plate and extend axially therealong.
 5. An acoustic liner according to claim 4, wherein:the core member is comprised of reinforced carbon fibers; and the core member is adhesively secured to each of the honeycomb structures.
 6. In a jet engine having an axially and circumferentially extending shroud defining an engine axis, a fan rotatably mounted inside the shroud, and a compressor and a turbine secured within the shroud, an acoustic liner circumferentially extending around the engine axis to attenuate sound waves generated in the engine, the acoustic liner comprising:a sound permeable inside plate circumferentially extending completely around the engine axis; a sound impermeable outside plate circumferentially extending completely around the engine axis, concentric with and radially spaced from the inside plate; a core member secured between the inside and outside plates, extending axially along and around the engine axis and forming a sine wave form in the circumferential direction around said axis, said sine wave form extending completely around the engine axis, wherein the core member and the inside plate form a multitude of varying depth sound absorption chambers to attenuate sound waves over a broad range of frequencies; and a multitude of honeycomb structures located in the sound absorption chambers to further attenuate the sound waves; and wherein the core member has an axial length; and in any plane perpendicular to the engine axis and along said axial length, the core member forms a sine wave form having a generally uniform height and wave length.
 7. An acoustic liner according to claim 6, wherein the honeycomb structure in each chamber circumferentially extends completely across the chamber, and radially extends completely between the inside plate and the core member to inhibit axial transmission of the sound waves inside the liner.
 8. An acoustic liner according to claim 7, wherein:each honeycomb structure is secured to both the inside plate and the core member; the core member is comprised of reinforced carbon fibers; and the core member is adhesively secured to each of the honeycomb structures. 